Superoxide and nitric oxide production within the outer medulla (OM) of the kidney are known to play an important role in sodium homeostasis and in salt-sensitive hypertension and renal injury. Little is yet known about the highly reactive H2O2 molecule which is involved in these processes and may account for as much as 50% of the hypertension and renal injury observed in the Dahl salt-sensitive (SS) rat. We hypothesize that increased NaCI delivery to the medullary thick ascending limb (mTAL) of the OM, as occurs following an increase in NaCI intake, stimulates mitochondrial production of H2O2 in the mTAL epithelial cells that diffuses to the cell membrane and enhances the activity of NADPH oxidase leading to an overall increase of cellular levels of H2O2 (Aim 1). We propose that this H2O2 response is significantly amplified in SS rats by greater expression of the p67phox cytosolic subunit of NADPH oxidase compared to a salt-resistant control strain as explored in Aim 2. The contribution of p67phox to these events will be determined ufilizing SS rats with a ubiquitous null mutation in the p67phox gene and salt-resistant SS.13BN26 rats with mTAL-specific transgenic overexpression of p67phox. In Aim 3, studies will determine if the greater production of H2O2 in the mTAL of SS rats results in diffusion of H2O2 into the interstitial space of the surrounding vasa recta which results in pericyte-mediated vasoconstriction and reduction of medullary blood flow leading to the initial moderate rise of arterial pressure. Aim 4 will determine if the rise of blood pressure with salt intake provokes renal T-lymphocyte infiltration, excess p67phox and fumaric acid in the mTAL leading to greater H2O2 production and a progression from a mild to severe form of hypertension and renal injury. This is a highly collaborative protocol between Projects 1, 2 and 3 that will utilize a computer-controlled system to examine the consequences of the elevated renal perfusion pressure by protecfing one kidney from the hypertension while the other is exposed to the elevated blood pressure. Since technical limitations have impeded a thorough mechanistic understanding of the role of H2O2 in renal function and hypertension, a number of new fluorescent imaging approaches, a novel fluorescent probe that specifically detects mitochondrial changes of H2O2, and a novel inhibitor of mitochondrial H2O2 will be utilized to advance our understanding of this field. Several novel genetically engineered rat model systems have been developed to test several of the key hypotheses including an SS rat with a ubiquitous null mutation in the p67phox gene and a salt-insensitive SS.13BN26 in which p67phox is transgenically overexpressed only in the thick ascending limb of Henle. H2O2 appears to be an important signaling molecule in the OM of the kidney. If more effective antioxidant therapies are to be developed it will require knowledge of the expression and regulation of the key pathways and enzymes responsible for H2O2 formation and their functional relevance at the level of the tissue and whole organism. By identifying the two novel controllers of H2O2 production, one related to the mitochondria and the other to the p67phox gene, we anticipate identifying new therapeutic targets for hypertension.